The present invention relates to a fuel supply system for fuel supply of an internal combustion engine from a fuel container by means of a fuel supply unit.
Fuel supply systems of the above mentioned general type are known in the art, as disclosed for example in the German patent document DE 42 42 242 A1. The fuel supply unit of such a system is arranged in the fuel container and conventionally has a pressure limiting valve with adjustable maximum pressure. A pump in the fuel supply unit pumps the fuel from the supply container into a fuel supply conduit. Conventionally, the operational pressure of the fuel supply conduit is lower than the maximum pressure which is monitored by the pressure limiting valve. If the maximum pressure is exceeded, the pressure limiting valve which is usually located in the upper region of the housing of the fuel supply unit, supplies back the excessive fuel into the fuel container. The operational pressure in the fuel supply conduit is controlled by an adjustable pressure regulating valve which discharges the excessive fuel into the fuel return conduit for maintaining a regulated operational pressure, so that in the internal combustion engine conventionally only the operational pressure acts.
The known fuel supply system has the disadvantage that the fuel supply pressure increase over the operational pressure is not possible without adjustment of the pressure regulating valve for the fuel in the fuel supply conduit. A short-term excess of the operational pressure in the fuel supply conduit, which in particular is required in a cold start phase in an internal combustion engine or in a warm start phase, is thereby possible only via an adjustment of the nominal value for the regulated operational pressure of the pressure regulating valve or by an electrically operating additional pump. During adjustment of the operational pressure in each starting phase, the regulated operational pressure must be adjusted. There is the disadvantage for the known fuel supply systems that the operational pressure in the fuel supply conduit is not exactly reproducable after each starting step. In another case, an electrically operating additional pump which is switched off after the starting phase, has the disadvantage of increased cost and increased construction, maintenance and operational expenses for the additional pump.